1. Field of the Invention
This invention relates to a light (diffusing, optical screen material such as may be used, for example, as a rear or front projection screen or as a depixelating screen for reducing the perception of individual pixels in a pixelated (e.g. LCD) display or, analogously, of tile individual scan lines of a CRT display.
2. Description of the Prior Art
The formulation and processing of polymer materials to create diffusers suitable as optical screen materials have been described in U.S. Pat. No. 2,287,556 (Land), U.S. Pat. No. 4,983,016 (Yamamoto), EP-A-0464499 (Sumitomo) and EP-A-0843203 (Nashua). These specifications describe symmetrically and asymmetrically diffusing optical screen materials which may be created by extrusion or casting. Asymmetry in diffusion is imparted typically by stretching to create orientation. However all the above materials appear xe2x80x9cwhitexe2x80x9d to a greater or lesser extent in ambient lighting conditions, that is without illumination from an imaging system. The whiteness is a function of light scattering by the dispersed particles incorporated in the materials to render them light-diffusing. As a result of this whiteness, an image created optically in the diffusing material suffers from a lack of contrast. Contrast may be recovered or improved for example, by the addition of a polarising film as described in the Sumitomo Japanese laid open Application Ser. No. 5-113606 or by the use of a second layer of a tinted acrylic material. Although both these approaches can improve contrast, this is achieved with a significant loss in brightness (gain), (as much as 50% where a polarising film is used).
It is an object of the present invention to provide a diffusing material providing improved contrast without a significant reduction in brightness.
According to one aspect of the invention there is provided a high contrast projection, depixelating, or the like screen, comprising a primary matrix of a first, transparent material, a first quantity of discrete bodies of a second transparent material of a refractive index different from the primary matrix material distributed in said primary matrix and a second quantity of discrete light absorbing or filtering bodies distributed in said primary matrix.
According to another aspect of the invention, there is provided a method of forming a screen according to the first-mentioned aspect, comprising compounding, in a molten or plastic state, a first light-transmitting thermoplastics matrix material with a second light-transmitting thermoplastics material insoluble in, and having a different refractive index from the first, and with a third thermoplastics material insoluble in the first, said third material being light-absorbing or attenuating, the method further comprising extruding the resulting compound through a slot
Preferably, particularly where a high density of the discrete bodies in the primary matrix is contemplated, the second and third plastics materials are also mutually incompatible (i.e. mutually insoluble).
According to yet another aspect of the invention there is provided a high contrast projection, depixelating, or the like screen, comprising discrete bodies of a first material and refractive index in a matrix of a second material and refractive index, one said material being transparent and the other being light-filtering.
According to still another aspect of the invention there is provided A method of forming a screen, comprising forming a mixture comprising a plurality of discrete light-transmitting bodies in a fluid, light-filtering matrix or binder, forming the resulting mixture into a thin layer or sheet, and causing or allowing at least said binder to set.